Conventionally, a welding device which automatically performs butt welding of tubes is known. For example, as shown in FIGS. 6A and 6B, Patent Literature 1 discloses a welding device 100 which is capable of performing all-position welding of tubes 150 placed to be oriented horizontally (laid down).
In the welding device 100, a welding torch 130 is capable of being moved in the radial direction of the tubes 150 in order to adjust the length of an arc. Specifically, the welding device 100 includes two rings of a circular-arc shape which are a rotational motion ring 110 and a vertical motion ring 120, as rotary members which are rotatable around the center axis of the tubes 150. The rotational motion ring 110 and the vertical motion ring 120 are rotatable independently of each other.
The welding torch 130 is pivotally mounted to the rotational motion ring 110 via a torch pedestal 140. The vertical motion ring 120 is provided with a groove 125 inclined with respect to a circumferential direction. A shaft 145 fixed to the torch pedestal 140 is inserted into the groove 125. The vertical motion ring 120 relatively rotates with respect to the rotational motion ring 110. Thus, the shaft 145 is guided by the groove 125, and the welding torch 130 is moved close to or away from the tubes 150.